Multi-antenna wireless communication systems have generally grown more advanced over time by the introduction of additional antennas. For example, previous versions of High Speed Downlink Packet Access (HSDPA) supported downlink transmission using up to two transmit antennas. Forthcoming versions of HSDPA, however, will support downlink transmission using up to four transmit antennas (also known as 4 branch MIMO). See, e.g., RP-111393, “New WI: Four Branch MIMO transmission for HSDPA,” and R1-111763, 4-branch MIMO for HSDPA. While introducing additional transmit antennas will advantageously increase the downlink performance of those wireless devices newly configured to receive transmissions from four transmit antennas, it threatens to actually decrease the downlink performance of other, legacy wireless devices.
More particularly, in order to support multi-antenna transmission (e.g., spatial multiplexing), a wireless device must obtain different channel estimates for each spatial layer, in order to individually characterize each layer. A device obtains such channel estimates by measuring a pilot channel transmitted from each antenna. Thus, the introduction of additional antennas leads to the introduction of additional pilot channels.
In more detail, pilot channels serve two main functions. First, a wireless device uses pilot channel estimates to estimate or otherwise determine so-called channel state information (CSI) estimation. CSI includes a recommended rank (i.e., the number of spatial layers), a channel quality indicator (CQI), and a precoding control indicator (PCI). Second, a wireless device uses pilot channel estimates to estimate the channel for demodulation purposes.
In HSPDA versions that will support 4 transmit antennas, two different approaches are possible for transmitting pilot channels. The first approach just employs so-called common pilot channels (one for each antenna) that are transmitted to all wireless devices in a cell, without device-specific beamforming. Devices measure these common pilot channels for both CSI determination and channel estimation for demodulation. The second approach, by contrast, employs common pilot channels for CSI determination, but employs dedicated pilot channels for estimating the channel for demodulation. A dedicated pilot channel is transmitted to a specific device and may be beamformed specifically for that device. Beamforming a pilot channel specifically for a device improves the orthogonality of the pilot channel with the HS-PDSCH (High-Speed Physical Downlink Shared Channel) and reduces the energy needed for the pilot channel.
Regardless of the particular approach, though, the threat to the downlink performance of legacy devices can arise if the newly introduced common pilot channels are transmitted (e.g., with high power) during time intervals in which legacy devices, which are not able to demodulate transmissions from four antennas, are scheduled. These legacy devices cannot make use of the energy in the newly introduced common pilot channels. Worse, the energy in the newly introduced common pilot channels will reduce the amount of energy available for HS-PDSCH scheduling to the legacy devices and act as interference which will detoriate the channel quality.
One approach to reducing the performance impacts to legacy devices is to “gate” newly introduced common pilot channels. Known techniques for gating a common pilot channel simply transmit the common pilot channel less frequently (i.e., occasionally “transmit” the common pilot with zero power as opposed to high power when the pilot is not gated).
The gating approach appears promising for reducing the performance degradation of legacy devices, but under some circumstances would counteract the performance improvements achieved for non-legacy devices via four antenna transmission. Indeed, CSI determination by non-legacy devices during gating would be less reliable, since the CSI determination would be based on only 2 common pilot channels rather than 4. Transmissions scheduled based on such CSI will likely fail, resulting in poor performance.
Another approach to reducing the performance impacts to legacy devices is to reduce to a low value the power of common pilot channels newly introduced for CSI determination, and to introduce other common pilot channels for data demodulation. For example, in some forthcoming 4TX HSDPA versions, 6 common pilot channels rather than just 4 will be transmitted to wireless devices in a cell without device-specific beamforming. In this case, though, 2 of the common pilot channels will be continuously transmitted with high power on two antennas and used by both legacy and non-legacy devices for CSI determination. Two other common pilot channels will be transmitted with low power on the other two antennas and used by only non-legacy devices for CSI determination. The 2 remaining common pilot channels will also be transmitted on the other two antennas, but will be transmitted with high power and used by non-legacy devices to estimate the channel for data demodulation. Because these remaining channels will be transmitted with high power, they will be transmitted less frequently, e.g., on an as needed basis. That is, these 2 remaining common pilot channels will be gated. More specifically, the 2 remaining common pilot channels will be gated by a base station when that base station is not transmitting data to at least one non-legacy wireless device configured to measure those 2 channels.
Introducing these 2 remaining common pilot channels will advantageously increase the downlink performance of those non-legacy wireless devices newly configured to receive those channels. But, when those 2 channels are being transmitted, they cause interference to the other 4 common pilot channels, meaning that a device (whether legacy or non-legacy) will not be able to report CSI as accurately using those 4 other common pilot channels. This interference is of course reduced to an extent by gating the 2 remaining common pilot channels, but the interference still proves unacceptable under some circumstances.